Spin Dynamical Properties of the Layered Perovskite La1.2Sr1.8Mn2O7

Inelastic neutron-scattering measurements were performed on a single crystal of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (Tc ~ 120K). We found that the spin wave dispersion is almost perfectly two-dimensional with the in-plane spin stiffness constant D ~ 151meVA. The value is similar to that of similarly doped La1-xSrxMnO3 though its Tc is three times higher, indicating a large renormalization due to low dimensionality. There exist two branches due to a coupling between layers within a double-layer. The out-of-plane coupling is about 30% of the in-plane coupling though the Mn-O bond lengths are similar.Comment: 3 pages, 3 figures J. Phys. Chem. Solids in pres

Magnon Damping by magnon-phonon coupling in Manganese Perovskites

Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures ($T_C$s) of different manganites, their low-temperature spin waves were found to have very similar dispersions with the zone boundary magnon softening. From the wavevector dependence of the magnon lifetime effects and its correlation with the dispersions of the optical phonon modes, we argue that a strong magneto-elastic coupling is responsible for the observed low temperature anomalous spin dynamical behavior of the manganites.Comment: 11 pages, 4 figure

Upgrade to the Birmingham Irradiation Facility

The Birmingham Irradiation Facility was developed in 2013 at the University of Birmingham using the Medical Physics MC40 cyclotron. It can achieve High Luminosity LHC (HL-LHC) fluences of 1015 (1 MeV neutron equivalent (neq)) cm-2 in 80 s with proton beam currents of 1 μA and so can evaluate effectively the performance and durability of detector technologies and new components to be used for the HL-LHC. Irradiations of silicon sensors and passive materials can be carried out in a temperature controlled cold box which moves continuously through the homogenous beamspot. This movement is provided by a pre-configured XY-axis Cartesian robot scanning system. In 2014 the cooling system and cold box were upgraded from a recirculating glycol chiller system to a liquid nitrogen evaporative system. The new cooling system achieves a stable temperature of -50 °C in 30 min and aims to maintain sub-0 °C temperatures on the sensors during irradiations. This paper reviews the design, development, commissioning and performance of the new cooling system

The complex multiferroic phase diagram of Mn1−x_{1-x}Cox_xWO4_4

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.Comment: 18 pages, 6 figure

Theory of anomalous magnon softening in ferromagnetic manganites

In metallic manganites with low Curie temperatures, a peculiar softening of the magnon spectrum close to the magnetic zone boundary has experimentally been observed. Here we present a theory of the renormalization of the magnetic excitation spectrum in colossal magnetoresistance compounds. The theory is based on the modulation of magnetic exchange bonds by the orbital degree of freedom of double-degenerate e_g electrons. The model considered is an orbitally degenerate double-exchange system coupled to Jahn-Teller active phonons which we treat in the limit of strong onsite repulsions. Charge and coupled orbital-lattice fluctuations are identified as the main origin of the unusual softening of the magnetic spectrum

Anomalous ferromagnetic spin fluctuations in an antiferromagnetic insulator Pr_{1-x}Ca_{x}MnO_{3}

The high temperature paramagnetic state in an antiferromagnetic (AFM) insulator Pr_{1-x}Ca_{x}MnO_{3} is characterized by the ferromagnetic (FM) spin fluctuations with an anomalously small energy scale. The FM fluctuations show a precipitous decrease of the intensity at the charge ordering temperature T_{CO}, but persist below T_{CO}, and vanish at the AFM transition temperature T_{N}. These results demonstrate the importance of the spin ordering for the complete switching of the FM fluctuation in doped manganites.Comment: REVTeX, 5 pages, 4 figures, submitted to Phys. Rev.

Low-temperature electrical transport and double exchange in La(Pb,Ca)MnO

The resistivity in the ferromagnetic state of flux-grown La_{2/3}(Pb,Ca)_{1/3}MnO_3 single crystals, measured in magnetic fields up to 7 T, reveals a strong quadratic temperature dependence at and above 50 K. At lower temperatures, this contribution drops precipitously leaving the resistivity essentially temperature independent below 20 K. The Seebeck coefficient also reflects a change of regime at the same temperature. We attribute this behavior to a cut-off of single magnon scattering processes at long wavelengths due to the polarized bands of a double-exchange ferromagnet.Comment: 10 pages, TeX, 4 figures. Revised version. Submitte

Structure and Spin Dynamics of La0.85_{0.85}Sr0.15_{0.15}MnO3_3

Neutron scattering has been used to study the structure and spin dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0, 0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$ dependence at low T, and the damping at small q follows $q^4T^{2}$. An anomalously strong quasielastic component, however, develops at small wave vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the $e_g$ electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript

Random Fan-Out State Induced by Site-Random Interlayer Couplings

We study the low-temperature properties of a classical Heisenberg model with site-random interlayer couplings on the cubic lattice. This model is introduced as a simplified effective model of Sr(Fe$_{1-x}$Mn$_{x}$)O$_2$, which was recently synthesized. In this material, when $x=0.3$, $(\pi\pi\pi)$ and $(\pi\pi0)$ mixed ordering is observed by neutron diffraction measurements. By Monte Carlo simulations, we find an exotic bulk spin structure that explains the experimentally obtained results. We name this spin structure the "random fan-out state". The mean-field calculations provide an intuitive understanding of this phase being induced by the site-random interlayer couplings. Since Rietveld analysis assuming the random fan-out state agrees well with the neutron diffraction pattern of Sr(Fe$_{0.7}$Mn$_{0.3}$)O$_2$, we conclude that the random fan-out state is reasonable for the spin-ordering pattern of Sr(Fe$_{0.7}$Mn$_{0.3}$)O$_2$ at the low-temperature phase.Comment: 13 pages, 12 figure

Spin Wave Theory of Double Exchange Ferromagnets

We construct the 1/S spin-wave expansion for double exchange ferromagnets at T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently large, resulting in a fully saturated, ferromagnetic half-metallic ground state. We evaluate corrections to the magnon dispersion law, and we also find that, in contrast to earlier statements in the literature, magnon-electron scattering does give rise to spin wave damping. We analyse the momentum dependence of these quantities and discuss the experimental implications for colossal magnetoresistance compounds.Comment: 4 pages, Latex-Revtex, 2 PostScript figures. Minor revisions, references added. See also cond-mat/990921